The present invention relates to the control or stabilisation of the lasing wavelength of a source of laser radiation, particularly the wavelength stabilisation of a semiconductor laser source.
The wavelength of laser radiation generated by semiconductor lasers needs to be stabilised in many applications, for example when the laser radiation is used in Wavelength Division Multiplexing (WDM) systems, where different wavelengths launched into a fibre must have tightly controlled wavelengths so as not to interfere one another. WDM systems may carry a number of channels with different wavelengths around 800 nm to 1600 nm, separated by about 20 nm. More recently, Dense Wavelength Division Multiplexing (DWDM) systems have appeared where channel separation is less than or about 2 nm.
A high degree of wavelength stabilisation is also useful in erbium-doped fibre amplification systems. Here, the wavelength of the pump laser needs to be tightly controlled to match the absorption characteristics of the erbium-doped fibre. See, for example, the disclosure in patent document U.S. Pat. No. 6,144,788.
Laser wavelength can change owing to ageing effects or from unwanted optical feedback into the laser resonant cavity. In many applications the temperature of the laser device can vary, for example due to varying electrical power dissipation within the device, or because of changes in ambient temperature. Temperature changes will, in general, affect the laser wavelength, for example by changing the optical length of the resonant cavity or by changing the gain characteristics of the lasing medium.
There is a particular problem with distributed feedback (DFB) lasers and other semiconductor lasers where a wavelength selective element or grating is buried in the semiconductor material, in that the grating pitch can change with temperature owing to a change in length of the device itself due to thermal expansion and changes in refractive index with temperature or ageing. In DWDM systems, tight control of each laser wavelength is a prerequisite for reliable system operation. Usually, such systems use fixed wavelength DFB lasers whose emission wavelength is controlled by the use of a temperature sensing element and a thermo-cooler. However, such active thermoelectric cooling of the semiconductor laser is expensive and inconvenient, and increases electrical power consumption and is not absolute.
One way of stabilising the lasing wavelength of a source of laser radiation, particularly a laser diode, is to couple the output from the laser into an optical fibre pigtail having a grating. Such gratings are normally alternating bands of relatively high and low index of refraction, aligned transversely to the propagation direction of the laser radiation within the fibre. The grating is fabricated to selectively reflect a particular characteristic wavelength within the fibre back to the laser source. If the amount of the optical feedback is sufficient, then the wavelength of the laser diode can be stabilised against fluctuations owing to temperature variations of the laser, short-term power variations, or long-term ageing effects.
Such fibre pigtail assemblies, however, require precision assembly and are relatively expensive to manufacture.
It is an object of the present invention to provide a more convenient method and apparatus for stabilising the wavelength of a source of laser radiation.
Accordingly, the invention provides a source of laser radiation, comprising a semiconductor material laser, a substrate to which the laser is mounted, a resonant optical cavity within the semiconductor material, the cavity having an active medium for generating laser radiation and one or more gaps in the semiconductor material within the cavity, wherein the substrate is deformable by the application of a mechanical stress to vary the size of said gap(s) in order to change the optical size of said gap(s) and hence to vary the wavelength of laser radiation generated by the semiconductor laser.
The semiconductor material may be a III-V material, for example formed from InP, and the active medium may be a quaternary semiconductor bulk or quantum well material.
The ability to vary the size of the gaps by the application of a mechanical stress to the substrate permits the generated wavelength to be controlled over a certain wavelength range, and this in turn allows the wavelength either to be stabilised to a particular value, or to be tuned over a certain range of wavelengths.
Therefore, the invention also provides a wavelength stabilised laser unit, comprising a source of laser radiation for generating wavelength-stabilised laser radiation, wherein the laser source is according to the invention.
The gaps may also cause the cavity to operate in a single longitudinal mode, particularly if there are a plurality of said gaps. This is particularly advantageous, as the gaps then fulfil two functions: both wavelength control or stabilisation and single wavelength selection.
The substrate may be a semiconductor material, a metal, a ceramic, or any other suitable material.
The gaps may be changed either passively, or actively under the action of an actuator, which may be controlled by a controller.
The stress may therefore be applied in a variety of ways. In one embodiment, the source may comprise a piezo-electric actuator arranged to apply said mechanical stress to the substrate. The applied stress may then cause the substrate and semiconductor laser to bow and hence vary the size of said gap(s).
Another way of applying the stress is by thermal expansion or contraction of the substrate and/or laser diode when the temperature of the substrate changes. For example, the substrate and semiconductor material may have differential coefficients of thermal expansion which are sufficient to cause the substrate and semiconductor laser to bow upon application of the mechanical stress. The bowing then causes the size of said gap(s) to vary.
The variation in the size of said gap(s) caused by said expansion or contraction of the substrate and/or semiconductor laser may, in some applications, be used to stabilise the wavelength of laser radiation generated by the semiconductor laser against changes owing to temperature variation or ageing of the semiconductor laser.
In either embodiment, the optical cavity may be between 100 xcexcm and 500 xcexcm long. The bowing of the semiconductor laser may then have a radius of between 30 mm and 120 mm at a first temperature, and essentially no bowing at a second temperature, said first and second temperatures being separated by about 100xc2x0 C.
A typical length of the gaps is between 0.2 and 2.0 xcexcm extending in a longitudinal direction with respect to the cavity.
A typical depth of the gaps is between 1 and 10 xcexcm deep in a direction transverse to a longitudinal direction with respect to the cavity.
In a preferred embodiment of the invention, the wavelength stabilised laser unit may comprise a controller for stabilising the wavelength of laser radiation generated by the laser source, and an actuator for applying a mechanical stress to the substrate. The actuator is then responsive to the controller in order to deform the substrate and so vary the size of said gap(s) and hence to stabilise the wavelength of laser radiation generated by the semiconductor laser.
The controller may be linked to a temperature sensor for sensing the temperature of the semiconductor laser and arranged to control the actuator in response to the temperature sensor in order to stabilise said wavelength.
The controller may be linked to a device directly measuring the laser wavelength such as a wavemeter, a wavelength sensitive filter, a spectrometer or other device which measures wavelength directly.
The invention also provides a method of varying the wavelength of a source of laser radiation, said source comprising a semiconductor material laser, a substrate to which the laser is mounted, a resonant optical cavity within the semiconductor material, the cavity having an active medium, and one or more gaps in the semiconductor material within the cavity, wherein the method comprises the steps of:
i) using the active medium to generate laser radiation;
ii) using said gap(s) to selectively apply feedback within the cavity to one or more particular wavelengths;
iii) deforming the substrate by the application of a mechanical stress to vary to size of said gap(s) and hence to vary the wavelength of laser radiation generated by the semiconductor laser.
The invention further provides a method of stabilising the wavelength of a laser unit, the laser unit comprising a semiconductor material laser, a substrate to which the semiconductor laser is mounted, a resonant optical cavity within the semiconductor material, the cavity having an active medium and one or more gaps in the semiconductor material within the cavity, a controller for stabilising the wavelength of laser radiation generated by the semiconductor laser, and an actuator for applying a mechanical stress to the substrate, wherein the method comprises the steps of:
i) using the active medium to generate laser radiation;
ii) using said gap(s) to selectively apply feedback within the cavity to one or more particular wavelengths of said radiation;
iii) controlling the actuator with the controller to deform the substrate by the application of said mechanical stress to vary to size of said gap(s) and hence to stabilise the wavelength of laser radiation generated by the semiconductor laser.
The semiconductor laser may be a vertical cavity structure, but the invention is particularly applicable to a horizontal cavity structure, such as an edge-emitting buried heterostructure device, owing to the fact that gaps can more readily be formed with such a device, for example by etching downwards into the laser structure from an exposed upper surface of the horizontal device.